Glass Selection
The correct choice of glass for a particular application requires the consideration of a number of different characteristics. For most installations, the following glass properties should be evaluated: color and appearance, visible light transmission and reflection (from both sides), solar transmission (Solar Heat Gain Coefficient) and absorption, thermal and acoustic insulation, strength, deflection under load, fire rating, electromagnetic shielding, and where applicable, code or safety requirements. Other properties such as flatness, durability and ease of cleaning/maintenance access, should also be considered.
MAJOR COMPONENTS OF GLASS
Silica 70-74%
Soda 12-15%
Lime 8-10%
Magnesium Oxide 3.5-4.5%
Potassium Oxide 0.3-0.8%
Alumina 0.0-2.0%
Iron Oxide 0.08-0.11%
GENERAL PROPERTIES OF GLASS
Refractive Index 1.50-1.58
Surface reflection 4% or each side (8%)
Softening point 720-730 Celsius
Specific Heat 0-100 Celsius 0.20
Compressive strength (25mm cube) 248 MPa
Tensile strength Annealed 19.3-28.4 MPa
Tensile strength Toughened 175 MPa
Coefficient of linear expansion (room temperature to 350 Celsius) 9.0 x 10-6/degree Celsius
Hardness Mohs' Scale 6.0
Density 2500 kg/m3
Youngs Modulus (Elasticity) 69 GPa
Possions' Ratio 0.23
BREAKAGE
Annealed float glass does not resist high stresses from the impact of an object. When broken it shatters into large sharp pieces
Laminated safety glass has approximately the same impact strength resistance as that of the equivalent thickness of annealed glass. If broken it remains intact on its PVB interlayer and shards do not fly out. Because of its high resistance to penetration, laminated glass is very safe and can be used in a wide variety of applications. Laminated glass should always be used for overhead glazing (<15 degree off vertical) to prevent risk of glass falling in the event of breakage. Special consideration should be given to point fixed roof glazing wherein toughened glass is normally required, but even when laminated is still a major risk post breakage as the PVB will fail at the point fixings. In this case, either an ionoplast polymer interlayer (SGP) or a heat strengthened glass should be used to avoid such risk.
Heat Strengthened glass is about twice as strong as annealed glass and is generally used as a protection against thermal breakage. It is not a Grade A safety glass. Thermal breakage occurs where annealed glass breaks due to excessive differences in temperature between the centre and edge of the glass. Tinted, reflective and coated glasses are all susceptible to thermal breakage. Factors which affect thermal breakage include climate, edge quality of the glass, panel size and thickness, edge cover, glazing frame materials and associated colours, external shading devices and internal shading device / spandrel panel zones.
Toughened safety glass is up to five times stronger than annealed glass and offers high impact resistance. If broken the whole panel of glass shatters into small pieces of blunt granules which are relatively safe.
COLOUR AND APPEARANCE
There is a wide choice of colors available in today’s market. The use of reflective Low E coatings such as Pilkington Eclipse Advantage™ Reflective Low-E on the second surface (room side) of an exterior light enhances the base glass color seen from the exterior. In general, glass colors are subtle and need to be carefully evaluated.
The viewing of a full-scale mock-up, observed on the actual building site, with the proper orientation, is the only fully satisfactory evaluation method. The apparent color of glass is the combination of the glass color (clear, blue, blue-green, green, bronze, grey, dark grey, or coated), plus the color of incident light (midday sun or sunset), plus the color of any objects seen through the glass (drapes, blinds or insulation), plus the color of reflected objects (sky, clouds, etc.). Clearly the total appearance will continuously change as these individual components change.
Combining different glasses in insulating and laminated units will typically change the overall color and appearance. Reflections in glass will change in appearance depending on the heat treatment to the glass, sealed insulating glass air space pressures and on the distances of the viewer and the reflected objects from the glass. The use of a mock-up will help evaluate these effects.
THICKNESS
Glass thickness is usually decided by the strength and deflection requirements. Note that the color and appearance of tinted glass will change with thickness because thicker glasses absorb more transmitted light.
While the building codes and ASTM E 1300 “Load Resistance Standard” dictate the glass thickness and type (annealed, heat strengthened or tempered) needed to meet specified loads for a finite breakage probability, there are no code limits on glass deflection.
It is a common opinion that center of glass deflections greater than ¾” (19 mm) relative to the undeflected glass plane will be aesthetically objectionable for typical glazing installations.
VISIBLE LIGHT TRANSMISSION
Interior daylight levels will be determined by this value. Residential applications generally require higher levels than in commercial buildings. Note that if a high light level is needed with solar control, then performance coatings on low iron tinted glass, can be used to give almost as much visible transmission as clear glass, while giving better solar control than bronze or grey glass. Increased solar control, by using reflective glasses, reduces the visible light transmitted.
SOLAR TRANSMISSION AND ABSORPTION
The Solar Heat Gain Coefficient (SHGC) is the best measure of how much solar energy is admitted through a glazed opening. The SHGC compares the total solar heat gain through the glazing in question to the solar energy shining on the glazed area.
Low SHGC values reduce the solar gain and save on air conditioning costs.
A similar but less accurate measure of heat gain is the Shading Coefficient (SC). The SC compares the solar energy admitted to that coming through a piece of 1/8” clear glass. Residential glazing can use solar gain to advantage in winter. A high SHGC is desirable to maximize free passive solar heat gain in buildings where heating costs are greater than air conditioning costs.
Solar absorption makes a glass hot and causes thermal stress which, when excessive, can cause breakage of annealed glass.
Reflective glasses also have solar absorption that cannot be ignored. Note that the visible and solar values, for transmission and reflection, usually differ from each other because glass absorbs differing amounts of energy at different wavelengths.
THERMAL INSULATION
The U-Factor measures the thermal conductivity (from air to air on each side) of a glazing. Lower U-Factors are achieved by multiple glazing layers and the use of low emissivity coatings. The reciprocal of the U-Factor equals the R-value which measures thermal resistance (the R-value measure is more often used in wall and roof materials).
A Low-E coated, sealed unit, with a U-Factor of 0.33 Btu/hr.sq ft.deg F has an R-value of 1/0.33 or 3.0 hr.sq ft.deg F/Btu.
While this is less than the R value of most wall and roof materials, the U-Factor and R-values alone do not indicate the daylight transmission and beneficial passive solar heat gain which only glass can offer.
ACOUSTIC INSULATION
Thicker (heavier) glass transmits less sound than thin glass. Thick glass is very effective at stopping low frequency traffic noise, while thinner laminated glass is effective at controlling the mid-range frequencies of human conversation, etc. A laminate of thick layers combines the best of both methods. A full analysis of acoustic responses at different frequencies may be required in some cases.
STRENGTH
The ASTM E 1300 Standard Practice for Determining Load Resistance of Glass in Buildings should be consulted for glass capacity under specified loads. The strength of glass can be doubled by heat-strengthening, or quadrupled by full-tempering. Note that
annealed glass suffers a static fatigue effect which makes it only half as strong under long term loads (aquarium loads, snow loads, etc.) as under short term loads, such as wind gusts.
Laminated glass is 50% to 100% as strong (depending on aspect ratio and framing details) as monolithic glass of the same overall thickness and size when subjected to short duration loads at room temperatures.
Symmetrical insulating glass, where both lights are the same thickness, can carry almost twice the uniform load, as one of the lights on its own.
Glass has a finite probability of breakage under load. Its strength cannot be exactly predicted. For this reason, good design practice, while attempting to prevent glass breakage, will also always consider the possibility and consequences of unanticipated
breakage.
Annealed and heat-strengthened glass will break into large pieces when the applied load exceeds the glass strength. Most of the pieces remain in a glazed opening unless further force is applied. Laminated glass requires a considerable load to force it from a glazed opening even when both lites are broken. It can, however, be pierced by dense, hard, falling objects. Tempered glass breaks into very small pieces which can easily fall from a glazed opening, and often fall in small clumps which do not separate until they hit the ground.
The application of scrim backing to spandrel glass or the use of lamination is required if broken tempered glass must be held in place. On rare occasions heat-treated (tempered and sometimes even heat-strengthened) glass can break spontaneously, without any applied load, from a particular microscopically small inclusion or stone. Such inclusions would not cause a problem with annealed glass or low stress, heat-strengthened glass. Good design practice will address this unlikely possibility.
DEFLECTION UNDER LOAD
Rectangular glazing, supported on all four edges, does not deflect linearly under load; e.g., doubling the load does not cause twice the deflection. Deflection values can be calculated from ASTM E 1300.
On large sizes, the glass thickness needed may be decided by deflection limits rather than glass strength. Note that equal
thicknesses of annealed, heat-strengthened and tempered all deflect the same amount under equal loads. Laminated glass, at room temperature, of the same thickness, will deflect about the same amount for short duration loads (seconds) but will behave as
sliding layers and deflect further under long duration loads (hours or days).
FIRE RATING
Ordinary glass does not have a fire rating. Traditionally, wired glass is used to retain broken glass and prevent the passage of flames during a fire.
EMS SHIELDING
Electromagnetic shielding is needed in high security embassy buildings and other applications. Grounded transparent electrically conductive clear coatings (Pilkington TEC™ Glass) can be used to create windows which prevent radio wave signals from
computers and communications equipment from passing through glass.
SAFETY
Applicable building codes and specifications may determine the glass choice for safety or legal reasons. The appropriate local and
regional building codes must always be checked and adhered to.
BLAST RESISTANT GLAZING
ASTM International has developed ASTM F 1642-04 “Standard Test Method for Glazing and Glazing Systems Subject to
Airblast Loading”. Similar to the GSA/ISC performance criteria it too establishes a testing method that evaluates levels of
protection. These criteria require that glazed window products meet performance levels that correspond to specific levels
of protection.
Hazard Rating:
No Hazard = Glazing fractures but is fully retained in the facility test frame or glazing system frame and the rear surface is unbroken.
Minimal Hazard = Glazing fractures and the total length of tears in the glazing plus the total length of pullout from the edge of the frame is less than 20 per cent of the glazing sight perimeter. Three or less perforations from glazing slivers and no fragment indents affect witness panel
Very Low Hazard = Glazing fractures and is located within one (1) meter of the original location. Three or less perforations from glazing slivers and no fragment indents affect witness panel
Low Hazard = Glazing fractures. Glazing fragments generally fall between one (1) meter and three (3) meters of the original location. < 10 perforations 50 cm below the bottom of the specimen and none of the perforations penetrate through the full thickness of the witness panel
Hign Hazard = Glazing fractures. Glazing fragments generally fall between one (1) meter and three (3) meters of the original location. > 10 perforations in the area of the witness panel and one or more fragments penetrate fully through the witness panel
Toughened (fully heat tempered) glass provides a degree of safety but not complete security and is therefore not recommended for external window or door use. It can, however, resist high blast pressures without damage provided it is well supported in a strong and rigid frame. When it does break, its fragments cause fewer injuries than plain glass shards. When used on its own it should have anti-shatter film applied.
Laminated glass offers a higher level of protection than toughened glass. The minimum overall thickness of laminated glass classed as blast resistant is 7.5mm, including a minimum polyvinylbutryal (pvb) interlayer thickness of 1.5mm. The laminated glass should be fixed in a frame designed to withstand the bending effects of a static load of 7kN per square metre over the complete area of the glazing and frame. The fixings of rebates to the frame and of frame to the structure should be designed to withstand a line load of 20kN per metre of perimeter.
These loads are broadly applicable, for nominal design without factors of safety, to most glazing systems incorporating 7.5mm laminated glass, but are based on requirements for panes of about a 2 square metre area. The loads should be factored up by 50% to match the increased blast resistance of smaller windows of about 1 square metre. The line load for fixings may be factored down by 25% to 15kN per metre of perimeter for windows of about 4 square metres overall area; but the 7kN per square metre should not be significantly reduced when designing the frames of large glazing systems.
Panes with an edge dimension of 1m or more should be provided with a frame having a glazing rebate of at least 35mm giving a bearing of 30mm. Greater protection may be provided by setting the pane in double-sided adhesive security glazing tape or ideally bonded in sealant.
If robust frames and deep rebates cannot be provided, a level of protection equivalent to anti-shatter film on plain glass with net curtains can be achieved using thinner laminated glass, e.g. 6.8mm thick.
In double glazing, the preferred standard is a 7.5mm laminated glass inner pane with a 6mm toughened glass outer pane in a robust frame with deep rebates.
The same design loads may be applied as mentioned above. Users may elect merely to limit the spread of flying glass fragments by using laminated glass in less robust, standard frames. In this case the laminated inner pane may be reduced to 6.8mm thick (with 0.76mm pvb), with a 4mm toughened outer layer where panes are less than about 2 square metres. Even with standard frames some
attention should be given to the strength of fixings of the frame to the surrounding structure. It is recommended that fixings should be designed to resist not less than 25% of the values given above, i.e. not less than 5kN per metre. Fixings should be at a maximum of 350mm centres.
MAJOR COMPONENTS OF GLASS
Silica 70-74%
Soda 12-15%
Lime 8-10%
Magnesium Oxide 3.5-4.5%
Potassium Oxide 0.3-0.8%
Alumina 0.0-2.0%
Iron Oxide 0.08-0.11%
GENERAL PROPERTIES OF GLASS
Refractive Index 1.50-1.58
Surface reflection 4% or each side (8%)
Softening point 720-730 Celsius
Specific Heat 0-100 Celsius 0.20
Compressive strength (25mm cube) 248 MPa
Tensile strength Annealed 19.3-28.4 MPa
Tensile strength Toughened 175 MPa
Coefficient of linear expansion (room temperature to 350 Celsius) 9.0 x 10-6/degree Celsius
Hardness Mohs' Scale 6.0
Density 2500 kg/m3
Youngs Modulus (Elasticity) 69 GPa
Possions' Ratio 0.23
BREAKAGE
Annealed float glass does not resist high stresses from the impact of an object. When broken it shatters into large sharp pieces
Laminated safety glass has approximately the same impact strength resistance as that of the equivalent thickness of annealed glass. If broken it remains intact on its PVB interlayer and shards do not fly out. Because of its high resistance to penetration, laminated glass is very safe and can be used in a wide variety of applications. Laminated glass should always be used for overhead glazing (<15 degree off vertical) to prevent risk of glass falling in the event of breakage. Special consideration should be given to point fixed roof glazing wherein toughened glass is normally required, but even when laminated is still a major risk post breakage as the PVB will fail at the point fixings. In this case, either an ionoplast polymer interlayer (SGP) or a heat strengthened glass should be used to avoid such risk.
Heat Strengthened glass is about twice as strong as annealed glass and is generally used as a protection against thermal breakage. It is not a Grade A safety glass. Thermal breakage occurs where annealed glass breaks due to excessive differences in temperature between the centre and edge of the glass. Tinted, reflective and coated glasses are all susceptible to thermal breakage. Factors which affect thermal breakage include climate, edge quality of the glass, panel size and thickness, edge cover, glazing frame materials and associated colours, external shading devices and internal shading device / spandrel panel zones.
Toughened safety glass is up to five times stronger than annealed glass and offers high impact resistance. If broken the whole panel of glass shatters into small pieces of blunt granules which are relatively safe.
COLOUR AND APPEARANCE
There is a wide choice of colors available in today’s market. The use of reflective Low E coatings such as Pilkington Eclipse Advantage™ Reflective Low-E on the second surface (room side) of an exterior light enhances the base glass color seen from the exterior. In general, glass colors are subtle and need to be carefully evaluated.
The viewing of a full-scale mock-up, observed on the actual building site, with the proper orientation, is the only fully satisfactory evaluation method. The apparent color of glass is the combination of the glass color (clear, blue, blue-green, green, bronze, grey, dark grey, or coated), plus the color of incident light (midday sun or sunset), plus the color of any objects seen through the glass (drapes, blinds or insulation), plus the color of reflected objects (sky, clouds, etc.). Clearly the total appearance will continuously change as these individual components change.
Combining different glasses in insulating and laminated units will typically change the overall color and appearance. Reflections in glass will change in appearance depending on the heat treatment to the glass, sealed insulating glass air space pressures and on the distances of the viewer and the reflected objects from the glass. The use of a mock-up will help evaluate these effects.
THICKNESS
Glass thickness is usually decided by the strength and deflection requirements. Note that the color and appearance of tinted glass will change with thickness because thicker glasses absorb more transmitted light.
While the building codes and ASTM E 1300 “Load Resistance Standard” dictate the glass thickness and type (annealed, heat strengthened or tempered) needed to meet specified loads for a finite breakage probability, there are no code limits on glass deflection.
It is a common opinion that center of glass deflections greater than ¾” (19 mm) relative to the undeflected glass plane will be aesthetically objectionable for typical glazing installations.
VISIBLE LIGHT TRANSMISSION
Interior daylight levels will be determined by this value. Residential applications generally require higher levels than in commercial buildings. Note that if a high light level is needed with solar control, then performance coatings on low iron tinted glass, can be used to give almost as much visible transmission as clear glass, while giving better solar control than bronze or grey glass. Increased solar control, by using reflective glasses, reduces the visible light transmitted.
SOLAR TRANSMISSION AND ABSORPTION
The Solar Heat Gain Coefficient (SHGC) is the best measure of how much solar energy is admitted through a glazed opening. The SHGC compares the total solar heat gain through the glazing in question to the solar energy shining on the glazed area.
Low SHGC values reduce the solar gain and save on air conditioning costs.
A similar but less accurate measure of heat gain is the Shading Coefficient (SC). The SC compares the solar energy admitted to that coming through a piece of 1/8” clear glass. Residential glazing can use solar gain to advantage in winter. A high SHGC is desirable to maximize free passive solar heat gain in buildings where heating costs are greater than air conditioning costs.
Solar absorption makes a glass hot and causes thermal stress which, when excessive, can cause breakage of annealed glass.
Reflective glasses also have solar absorption that cannot be ignored. Note that the visible and solar values, for transmission and reflection, usually differ from each other because glass absorbs differing amounts of energy at different wavelengths.
THERMAL INSULATION
The U-Factor measures the thermal conductivity (from air to air on each side) of a glazing. Lower U-Factors are achieved by multiple glazing layers and the use of low emissivity coatings. The reciprocal of the U-Factor equals the R-value which measures thermal resistance (the R-value measure is more often used in wall and roof materials).
A Low-E coated, sealed unit, with a U-Factor of 0.33 Btu/hr.sq ft.deg F has an R-value of 1/0.33 or 3.0 hr.sq ft.deg F/Btu.
While this is less than the R value of most wall and roof materials, the U-Factor and R-values alone do not indicate the daylight transmission and beneficial passive solar heat gain which only glass can offer.
ACOUSTIC INSULATION
Thicker (heavier) glass transmits less sound than thin glass. Thick glass is very effective at stopping low frequency traffic noise, while thinner laminated glass is effective at controlling the mid-range frequencies of human conversation, etc. A laminate of thick layers combines the best of both methods. A full analysis of acoustic responses at different frequencies may be required in some cases.
STRENGTH
The ASTM E 1300 Standard Practice for Determining Load Resistance of Glass in Buildings should be consulted for glass capacity under specified loads. The strength of glass can be doubled by heat-strengthening, or quadrupled by full-tempering. Note that
annealed glass suffers a static fatigue effect which makes it only half as strong under long term loads (aquarium loads, snow loads, etc.) as under short term loads, such as wind gusts.
Laminated glass is 50% to 100% as strong (depending on aspect ratio and framing details) as monolithic glass of the same overall thickness and size when subjected to short duration loads at room temperatures.
Symmetrical insulating glass, where both lights are the same thickness, can carry almost twice the uniform load, as one of the lights on its own.
Glass has a finite probability of breakage under load. Its strength cannot be exactly predicted. For this reason, good design practice, while attempting to prevent glass breakage, will also always consider the possibility and consequences of unanticipated
breakage.
Annealed and heat-strengthened glass will break into large pieces when the applied load exceeds the glass strength. Most of the pieces remain in a glazed opening unless further force is applied. Laminated glass requires a considerable load to force it from a glazed opening even when both lites are broken. It can, however, be pierced by dense, hard, falling objects. Tempered glass breaks into very small pieces which can easily fall from a glazed opening, and often fall in small clumps which do not separate until they hit the ground.
The application of scrim backing to spandrel glass or the use of lamination is required if broken tempered glass must be held in place. On rare occasions heat-treated (tempered and sometimes even heat-strengthened) glass can break spontaneously, without any applied load, from a particular microscopically small inclusion or stone. Such inclusions would not cause a problem with annealed glass or low stress, heat-strengthened glass. Good design practice will address this unlikely possibility.
DEFLECTION UNDER LOAD
Rectangular glazing, supported on all four edges, does not deflect linearly under load; e.g., doubling the load does not cause twice the deflection. Deflection values can be calculated from ASTM E 1300.
On large sizes, the glass thickness needed may be decided by deflection limits rather than glass strength. Note that equal
thicknesses of annealed, heat-strengthened and tempered all deflect the same amount under equal loads. Laminated glass, at room temperature, of the same thickness, will deflect about the same amount for short duration loads (seconds) but will behave as
sliding layers and deflect further under long duration loads (hours or days).
FIRE RATING
Ordinary glass does not have a fire rating. Traditionally, wired glass is used to retain broken glass and prevent the passage of flames during a fire.
EMS SHIELDING
Electromagnetic shielding is needed in high security embassy buildings and other applications. Grounded transparent electrically conductive clear coatings (Pilkington TEC™ Glass) can be used to create windows which prevent radio wave signals from
computers and communications equipment from passing through glass.
SAFETY
Applicable building codes and specifications may determine the glass choice for safety or legal reasons. The appropriate local and
regional building codes must always be checked and adhered to.
BLAST RESISTANT GLAZING
ASTM International has developed ASTM F 1642-04 “Standard Test Method for Glazing and Glazing Systems Subject to
Airblast Loading”. Similar to the GSA/ISC performance criteria it too establishes a testing method that evaluates levels of
protection. These criteria require that glazed window products meet performance levels that correspond to specific levels
of protection.
Hazard Rating:
No Hazard = Glazing fractures but is fully retained in the facility test frame or glazing system frame and the rear surface is unbroken.
Minimal Hazard = Glazing fractures and the total length of tears in the glazing plus the total length of pullout from the edge of the frame is less than 20 per cent of the glazing sight perimeter. Three or less perforations from glazing slivers and no fragment indents affect witness panel
Very Low Hazard = Glazing fractures and is located within one (1) meter of the original location. Three or less perforations from glazing slivers and no fragment indents affect witness panel
Low Hazard = Glazing fractures. Glazing fragments generally fall between one (1) meter and three (3) meters of the original location. < 10 perforations 50 cm below the bottom of the specimen and none of the perforations penetrate through the full thickness of the witness panel
Hign Hazard = Glazing fractures. Glazing fragments generally fall between one (1) meter and three (3) meters of the original location. > 10 perforations in the area of the witness panel and one or more fragments penetrate fully through the witness panel
Toughened (fully heat tempered) glass provides a degree of safety but not complete security and is therefore not recommended for external window or door use. It can, however, resist high blast pressures without damage provided it is well supported in a strong and rigid frame. When it does break, its fragments cause fewer injuries than plain glass shards. When used on its own it should have anti-shatter film applied.
Laminated glass offers a higher level of protection than toughened glass. The minimum overall thickness of laminated glass classed as blast resistant is 7.5mm, including a minimum polyvinylbutryal (pvb) interlayer thickness of 1.5mm. The laminated glass should be fixed in a frame designed to withstand the bending effects of a static load of 7kN per square metre over the complete area of the glazing and frame. The fixings of rebates to the frame and of frame to the structure should be designed to withstand a line load of 20kN per metre of perimeter.
These loads are broadly applicable, for nominal design without factors of safety, to most glazing systems incorporating 7.5mm laminated glass, but are based on requirements for panes of about a 2 square metre area. The loads should be factored up by 50% to match the increased blast resistance of smaller windows of about 1 square metre. The line load for fixings may be factored down by 25% to 15kN per metre of perimeter for windows of about 4 square metres overall area; but the 7kN per square metre should not be significantly reduced when designing the frames of large glazing systems.
Panes with an edge dimension of 1m or more should be provided with a frame having a glazing rebate of at least 35mm giving a bearing of 30mm. Greater protection may be provided by setting the pane in double-sided adhesive security glazing tape or ideally bonded in sealant.
If robust frames and deep rebates cannot be provided, a level of protection equivalent to anti-shatter film on plain glass with net curtains can be achieved using thinner laminated glass, e.g. 6.8mm thick.
In double glazing, the preferred standard is a 7.5mm laminated glass inner pane with a 6mm toughened glass outer pane in a robust frame with deep rebates.
The same design loads may be applied as mentioned above. Users may elect merely to limit the spread of flying glass fragments by using laminated glass in less robust, standard frames. In this case the laminated inner pane may be reduced to 6.8mm thick (with 0.76mm pvb), with a 4mm toughened outer layer where panes are less than about 2 square metres. Even with standard frames some
attention should be given to the strength of fixings of the frame to the surrounding structure. It is recommended that fixings should be designed to resist not less than 25% of the values given above, i.e. not less than 5kN per metre. Fixings should be at a maximum of 350mm centres.